Quonset type greenhouses are typically constructed with a horizontal ridge pole at the top along the center of the greenhouse with at least two parallel purlins or purlin frame members spaced apart on each side of the center ridge pole. Additional purlins may be added for hanging baskets, increased greenhouse size, etc. The purlins are generally round galvanized steel or aluminum tubes or conduits in unit lengths, for example 20 feet (6 m) long, joined together. The quonset frame members and purlins are covered with one or more layers of sheet plastic such as polyethylene plastic for the "greenhouse effect" covering. The inner covering or skin rests with direct contact on the purlins. The greenhouse is typically heated to 60.degree. to 70.degree. F. (15.degree.-21.degree. C.) inside.
In northern climates with outside temperatures down to 40.degree. F. (5.degree. C.) and lower, condensation becomes a major factor. Moisture from the high humidity inside the greenhouse condenses on the inside surface of the covering and flows down the inclined walls. Anti-beading water-repellent coating such as "Sunclear" (TM) coating on the plastic sheet covering prevents water from beading and enhances the flow. The condensation flowing down the walls encounters the purlins. Continuous dripping from the purlins presents a widespread problem.